The present invention relates to a method according to the preamble of claim 1.
Vector control containing no motion sensor is a manner of controlling electric motor drives fed by a frequency converter which is about to become a standard solution. It provides a vast majority of applications with sufficient performance without the drawbacks caused by velocity feedback, including e.g. cost inflicted by the encoder used for feedback, mounting and cabling costs as well as service and maintenance.
Typically, vector control without motion sensors is based on measuring two or three output phase currents of a frequency converter. The measurement is implemented e.g. by current transducers based on the Hall effect, in which case the costs, need for space and the number of components of the method are extensive in connection with low-power frequency converters in particular. Making current measurement a simpler process enables savings to be achieved in terms of costs, need for space as well as the number of components; however, maintaining the performance level of the control method becomes a challenge.
In vector control, a reference value is calculated for a voltage vector so as to achieve a certain electromagnetic state for a motor to be controlled. A voltage reference determines the direction and magnitude of the voltage vector necessary during a modulation sequence. Based on this information, a modulator calculates switch references, i.e. the times for the states of each power switch used during a modulation sequence. An inverter part of a three-phase frequency converter comprises three pairs of switches, each switch pair being coupled in series between a positive and a negative busbar of a voltage intermediate circuit of the frequency converter. A point between the switch pairs constitutes a phase output of an inverter such that each phase may provide the output either with positive or negative voltage of the intermediate circuit.
In the case of vector control, the output voltage provided by switches is usually regarded as a complex-plane voltage vector. Switch pairs may be used for forming six voltage vectors which deviate from zero and which reside in a complex plane at a mutual 60 degree phase shift such that by coupling the output of phase A to be positive and the outputs of other phases B, C to be negative, a voltage vector +−− is obtained which resides in a direction parallel to a positive real axis of the complex plane, as shown in FIG. 1. Other voltage vectors are designated in a similar manner, e.g. a voltage vector −+− is a vector obtained when the output of phase B is coupled to a positive busbar of the voltage intermediate circuit while the output of other phases A, C is coupled to a negative busbar thereof. In connection with a three-phase frequency converter it is possible to produce eight voltage vectors, two of which being zero vectors +++ or −−− that are formed by coupling the outputs of each of them either to a positive busbar (+++) or to a negative busbar (−−−). In vector control, a voltage reference is implemented by calculating the time each switch combination is to be used in order to achieve the voltage reference during a modulation sequence.
In a conventional three-phase modulation shown in FIG. 2, switchings are carried out in every three phases during each modulation sequence. A modulation sequence starts from one zero vector and ends at the same zero vector, passing via another zero vector in a middle of the modulation sequence.
The simplest presently conceivable manner of measuring current so as to ensure the operation of vector control is to measure the current passing through the positive or the negative busbar of an intermediate circuit. This DC current measurement can be implemented e.g. by means of a shunt resistance situated in a busbar of the intermediate circuit, whose voltage drop is proportional to the current passing through the busbar. All current to an inverter part of a frequency converter passes via the intermediate circuit, which means that by measuring the current of the intermediate circuit, the current of one phase that is flowing to the load at a given moment is achieved. In addition to simplicity and the resulting inexpensiveness, need for less space and the small number of components, DC current measurement enables short-circuit protection to be implemented without any additional measurement electronics.
As far as DC current measurement is concerned, the conventional three-phase modulation method disclosed above is problematic, since both at a beginning and at an end of a modulation sequence as well as in a middle thereof, a zero vector is used during which the DC current is zero in size and contains no phase current information. In order to obtain phase current information, DC current sampling should take place at a moment in dependence on a modulation index, and thus changing from a modulation sequence to another, so that a voltage vector deviating from zero would then be in use and phase current information would thus exist, which, as far as the implementation in practice is concerned, would be problematic. Current measurement may also be implemented such that DC current is sampled at a high frequency, relying on getting a necessary number of phase currents measured in order to maintain reliability. However, such a method requires numerous samples to be taken and a considerable processing capacity in order to allocate these samples into currents of different phases on the basis of switch positions, for example.